This invention relates to apparatus for minimizing audible knock in an internal combustion engine by retarding spark timing, when necessary, from the normally set spark timing in response to the signal from an engine mounted vibration sensor. The use of such apparatus may allow an increased engine compression ratio, decreased fuel octane or the use of a turbocharger to boost engine output power and fuel economy, since the system responds to knock to keep it at a low level below audibility and far below any level that might cause harm to the engine.
A particularly difficult problem in such apparatus is the separation of vibrations due to knock, especially at a low level, from all the other vibrations present in an operating internal combustion engine and the generation of a signal accurately representing the intensity of those knock-induced vibrations. Various methods and types of apparatus have been proposed in the prior art for generating a suitable knock signal, but none have been found completely satisfactory in real time automotive engine control.
One technique that has been proposed is frequency discrimination, which takes advantage of the fact that knock has been found to produce vibrations at one or more particular characteristic frequencies, at least in a particular type of engine. It has been suggested that the output of an engine mounted vibration sensor be modified by a bandpass filter tuned to the particular characteristic frequency associated with knock in that engine. Unfortunately, however, this technique by itself has not generally proven to be sufficient in detecting the low levels of knock required, since there is usually noise at the characteristic frequency which is not knock related.
Another technique is amplitude discrimination, in which apparatus is provided for generating a unidirectional noise level reference signal from the sensor output and providing this signal and the sensor output signal to the two inputs of a comparator. The unidirectional noise level reference signal thus masks most of the noise in the sensor output signal so that only the peaks in the sensor output signal rising above the reference level, which are assumed to be mostly due to knock vibrations, contribute to the signal. This technique results in an improvement in signal to noise ratio over the frequency discrimination technique.
Since the amplitude discrimination technique, however, does not distinguish between a high amplitude vibration due to knock and one due to some other cause such as valve noise, the combination of frequency and amplitude discrimination techniques appears to be more practical than either technique alone. Such a combination provides for the sensor signal to be applied through a bandpass filter to a comparator which is also provided with a noise level reference.
In knock signal generating apparatus of the type described above, it has been found that some types of engine mounted vibration sensors tend to resonate at one or more particular frequencies and that these sensors can be at least approximately tuned to the characteristic frequency of vibrations due to knock. Thus the output signal of such a sensor would be already partially filtered and would require less electronic filtering before being applied to the comparator apparatus. Unfortunately, due to such factors as manufacturing tolerances and sensor aging characteristics, sensor-to-sensor variations may appear in the sensor tuning which will produce a wide variation in the output signal level of those vibrations at the design-specified characteristic frequency, although the average output signal broadband noise levels of the various sensors may be consistent. Therefore, the knock signal, which may be obtained by comparing the filtered sensor output signal containing knock-induced peaks at the characteristic frequency to a reference derived from the average broadband noise level, may indicate different knock levels for the same engine knock with different vibration sensors or with the same sensor over time.
The consistency of the apparatus for different sensors can be improved by deriving the noise reference signal from the output of the bandpass filter rather than the output of the sensor, so that the noise reference signal represents noise at the characteristic frequency only and not broadband noise. It is apparent that, if a sensor changes from being tuned precisely to the characteristic frequency to being tuned to some other frequency, the noise reference level at the characteristic frequency will change in the same proportion as the knock-induced output peaks at the characteristic frequency; therefore, the apparatus will produce more consistent results from sensors with varying tuned frequencies.
However, when knock is present, the strong peaks of knock-induced vibrations at the characteristic frequency tend to raise the noise reference and thus reduce the apparent amplitude of knock as seen by the comparator. This effect is magnified by the derivation of the DC reference level from the output of the bandpass filter, since noise components at other frequencies are attenuated with respect to suck knock-induced peaks. This effect may become great enough, when the noise reference level is derived from the bandpass filter output, to affect the ability of the system to respond to strong knock and allow knock to reach a level objectionable to the vehicle operator.